The coating of steel with protective metals such as zinc or aluminum is an economical means of providing corrosion resistance to various grades of steel. Hot dipping of steel is one of the most economical processes for mass production of coated steels and has increased dramatically over the pasta. As a result of this increased demands, the need for greater manufacturing efficiency in the galvanizing process has also gained prominence. Numerous projects have investigated the molten metal corrosion aspects of materials utilized for the submerged pot rolls and other hardware of continuous sheet galvanizing operations. The degradation and frequent failure of these structures results in significant production downtime and leads to high maintenance costs due to extensive repair and replacement.
The corrosion of submerged hardware materials by reacting with molten zinc-aluminum alloy have been studied in the past by analyzing weight loss and dimensional changes. Based on this analysis, an average corrosion rate is calculated. However, the instantaneous corrosion rate of materials attacked by the molten metal has not been analyzed.
Electrochemistry-based measurements aye powerful tools for studying the corrosion behaviors of materials in a service environment. Electrochemistry measurements using water-based solutions, acids, bases, and other chemicals acting as electrolytes at or near room temperature are well known. By analyzing the linear Tafel zone that is one of results of these measurements, the corrosion current density can be calculated and the corrosion rate can be deduced. Conventionally, a variety of electrochemical methods can be used to explore electrochemical reactions at the interface between the metal and an electrolyte solution. However, those methods are limited by the selection of working temperature, usually at or near room temperature (approximately 25° Centigrade (C)), as well as the electrolyte candidates, usually all types of water solutions.
The use of high temperature sensors which utilize electromotive force (EMF) measurements to detect the concentration of a specific component in a certain type of molten metal has been reported. For example, an aluminum sensor that detects the aluminum content in a zinc-aluminum bath, can use a KCl—NaCl liquid operating at about 460° C. as the electrolyte, or, in a subsequent improvement, the eutectic mixture of MgCl2—NaCl—KCl (in a liquid state as a molten salt), with the addition of 2-5 mol % AlCl3 (in liquid state) operating at about 460° C. Using the same principle, an aluminum sensor has been developed using a mixture of NaCl—AlCl3 (liquid) saturated with NaCl (solid) as the electrolyte operating at about 460° C. However, such sensors have a disposable and limited life because the reaction of AlCl3 with moisture or the evaporation of AlCl3 during use. In addition, the current design of aluminum sensors requires a beta-alumina solid electrolyte tube to be inserted into the NaCl—AlCl3 (liquid) saturated with NaCl (solid) electrolyte to prevent diffusion of ZnCl2 (liquid) to the reference electrode. This requirement complicates the sensor design, since an outer tube is required to contain the NaCl—AlCl3 (liquid) saturated with NaCl (solid) electrolyte, making it practically impossible to contain sufficient molten salt electrolyte to run the sensor for, a long time. Moreover, given the large volume required for the aluminum sensor, space restrictions limit the amount of the molten chloride electrolyte that can be used in the sensor. Therefore, current high temperature sensors are not able to carry out the electrochemical tests such as polarization and AC impedance performances.
There is a need for a system and method to study the in-situ behavior of molten metal instant corrosion rates and interfacial performance.